1. Field of the Invention
The invention relates to compositions of carbide-based and oxycarbide-based nanorods, carbon nanotubes including carbide and/or oxycarbide compounds, rigid porous structures including these compositions, and methods of making and using the same. More specifically, the invention relates to rigid three dimensional structures comprising carbon nanotubes bearing carbides and oxycarbides, carbide and/or oxycarbide-based nanorods having high surface areas and porosities, low bulk densities, substantially no micropores and increased crush strengths. The invention also relates to using the compositions of carbide-based nanorods, oxycarbide-based nanorods, carbon nanotubes comprising carbide and oxycarbide compounds and the rigid porous structures including these compositions as catalysts and catalyst supports, useful for many types of heterogenous catalytic reactions frequently encountered in petrochemical and refining processes.
2. Description of the Related Art
Heterogeneous catalytic reactions are widely used in chemical processes in the petroleum, petrochemical and chemical industries. Such reactions are commonly performed with the reactant(s) and product(s) in the fluid phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between phases, i.e., the interface between the fluid phase of the reactant(s) and product(s) and the solid phase of the supported catalyst. Hence, the properties of the surface of a heterogeneous supported catalyst are significant factors in the effective use of that catalyst. Specifically, the surface area of the active catalyst, as supported, and the accessibility of that surface area to reactant chemiabsorption and product desorption are important. These factors affect the activity of the catalyst, i.e., the rate of conversion of reactants to products. The chemical purity of the catalyst and the catalyst support have an important effect on the selectivity of the catalyst, i.e., the degree to which the catalyst produces one product from among several products, and the life of the catalyst.
Generally catalytic activity is proportional to catalyst surface area. Therefore, a high specific area is desirable. However, that surface area must be accessible to reactants and products as well as to heat flow. The chemiabsorption of a reactant by a catalyst surface is preceded by the diffusion of that reactant through the internal structure of the catalyst.
Since the active catalyst compounds are often supported on the internal structure of a support, the accessibility of the internal structure of a support material to reactant(s), product(s) and heat flow is important. Porosity and pore size distribution of the support structure are measures of that accessibility. Activated carbons and charcoals used as catalyst supports have surface areas of about 1000 square meters per gram and porosities of less than one milliliter per gram. However, much of this surface area and porosity, as much as 50%, and often more, is associated with micropores, i.e., pores with pore diameters of 2 nanometers or less. These pores can be inaccessible because of diffusion limitations. They are easily plugged and thereby deactivated. Thus, high porosity material where the pores are mainly in the mesopore ( greater than 2 nanometers) or macropore ( greater than 50 nanometers) ranges are most desirable.
It is also important that self-supported catalysts and supported catalysts not fracture or attrit during use because such fragments may become entrained in the reaction stream and must then be separated from the reaction mixture. The cost of replacing attritted catalyst, the cost of separating it from the reaction mixture and the risk of contaminating the product are all burdens upon the process. In other processes, e.g. where the solid supported catalyst is filtered from the process stream and recycled to the reaction zone, the fines may plug the filters and disrupt the process. It is also important that a catalyst, at the very least, minimize its contribution to the chemical contamination of reactant(s) and product(s). In the case of a catalyst support, this is even more important since the support is a potential source of contamination both to the catalyst it supports and to the chemical process. Further, some catalysts are particularly sensitive to contamination that can either promote unwanted competing reactions, i.e., affect its selectivity, or render the catalyst ineffective, i.e., xe2x80x9cpoisonxe2x80x9d it. Charcoal and commercial graphites or carbons made from petroleum residues usually contain trace amounts of sulfur or nitrogen as well as metals common to biological systems and may be undesirable for that reason.
Since the 1970s carbon nanofibers or nanotubes have been identified as materials of interest for such applications. Carbon nanotubes exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Nanofibers such as fibrils, bucky tubes and nanotubes are distinguishable from continuous carbon fibers commercially available as reinforcement materials. In contrast to nanofibers, which have, desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (LID) of at least 104 and often 106 or more. The diameter of continuous fibers is also far larger than that of nanofibers, being always  greater than 1.0xcexc and typically 5 to 7xcexc.
U.S. Pat. No. 5,576,466 to Ledoux et al. discloses a process for isomerizing straight chain hydrocarbons having at least seven carbon atoms using catalysts which include molybdenum compounds whose active surface consists of molybdenum carbide which is partially oxidized to form one or more oxycarbides. Ledoux et al. disclose several ways of obtaining an oxycarbide phase on molybdenum carbide. However, their methods require the formation of molybdenum carbides by reacting gaseous compounds of molybdenum metal with charcoal at temperatures between 900xc2x0 C. and 1400xc2x0 C. These are energy intensive processes. Moreover, the resulting molybdenum carbides have many similar drawbacks as other catalysts prepared with charcoal. For example, much of the surface area and porosity of the catalysts is associated with micropores and as such these catalysts are easily plugged and thereby deactivated.
While activated charcoals and other materials have been used as catalysts and catalyst supports, none have heretofore had all of the requisite qualities of high surface area porosity, pore size distribution, resistance to attrition and purity for the conduct of a variety of selected petrochemical and refining processes. For example, as stated above, although these materials have high surface area, much of the surface area is in the form of inaccessible micropores (i.e., diameter  less than 2 nm).
It would therefore be desirable to provide a family of catalysts and catalyst supports that have high accessible surface area, high porosity, resistance to attrition, are substantially free of micropores, are highly active and selective and show no significant deactivation after many hours of operation.
Nanofiber mats, assemblages and aggregates have been previously produced to take advantage of the increased surface area per gram achieved using extremely thin diameter fibers. These structures are typically composed of a plurality of intertwined or intermeshed nanotubes.
It is an object of the present invention to provide a composition including a multiplicity of oxycarbide nanorods having predominately diameters between 2.0 nm and 100 nm.
It is a further object of the present invention to provide another composition including a multiplicity of carbide nanorods comprising oxycarbides.
It is a further object of the present invention to provide another composition including a multiplicity of carbon nanotubes which have predominantly diameters between 2.0 nm and 100 nm, which nanotubes comprise carbides and optionally also oxycarbides.
It is a further object of the present invention to provide another composition including a multiplicity of carbon nanotubes having a carbide portion and optionally an oxycarbide portion.
It is a further object of the present invention to provide rigid porous structures which comprise compositions including a multiplicity of oxycarbide nanorods or a multiplicity of carbide nanorods with or without oxycarbides.
It is a further object of the present invention to provide compositions of matter which comprise three-dimensional rigid porous structures including oxycarbide nanorods, carbide nanorods, carbide nanorods comprising oxycarbides, or carbon nanotubes comprising a carbide portion and optionally an oxycarbide portion.
It is a further object of the present invention to provide methods for the preparation of and using the rigid porous structures described above.
It is still a further object of the invention to provide improved catalysts, catalyst supports and other compositions of industrial value based on composition including a multiplicity of carbide nanorods, oxycarbide nanorods and/or carbon nanotubes comprising carbides and oxycarbides.
It is still a further object of the invention to provide improved catalysts, catalyst supports and other compositions of industrial value based on three-dimensional rigid carbide and/or oxycarbide porous structures of the invention.
It is an object of the invention to provide improved catalytic systems, improved catalyst supports and supported catalysts for heterogenous catalytic reactions for use in chemical processes in the petroleum, petrochemical and chemical industries.
It is a further object of the invention to provide improved methods for preparing catalytic systems and supported catalysts.
It is another object of the invention to improve the economics and reliability of making and using catalytic systems and supported catalysts.
It is still a further object of the invention to provide improved, substantially pure, rigid carbide catalyst support of high porosity, activity, selectivity, purity and resistance to attrition.
The foregoing and other objects and advantages of the invention will be set forth in or will be apparent from the following description and drawings.
The present invention which addresses the needs of the prior art provides a composition including nanorods which contain oxycarbides. Another composition provided by the present invention includes carbide-based nanorods which also contain oxycarbides. Another composition provided by the invention relates to carbon nanotubes which bear both carbides and oxycarbides. In one composition the carbides retain the structure of the original aggregates of carbon nanotubes. However, a composition is also provided which includes carbide-based nanorods where the morphology of the aggregates of carbon nanotubes is not retained. The invention also provides a composition of carbides supported on carbon nanotubes where only a portion of the carbon nanotubes have been converted to carbide-based nonorods and/or carbides.
The present invention also provides rigid porous structures including oxycarbide nanorods and/or carbide-based nonorods and/or carbon nanotubes bearing carbides and oxycarbides. Depending on the morphology of the carbon nanotubes used as a source of carbon, the rigid porous structures can have a uniform or nonuniform pore distribution. Extrudates of oxycarbide nanorods and/or carbide-based nanorods and/or carbon nonotubes bearing oxycarbides and/or carbides are also provided. The extrudates of the present invention are glued together to form a rigid porous structure.
The invention also provides for the compositions and rigid porous structures of the invention to be used either as catalysts and/or catalyst supports in fluid phase catalytic chemical reactions.
The present invention also provides methods of making oxycarbide-based nanorods, carbide-based nanorods bearing oxycarbides and carbon-nanotubes bearing carbides and oxycarbides. Methods of making rigid porous structures are also provided. Rigid porous structures of carbide-nonorods an be formed by treating rigid porous structures of carbon nanotubes with a Q-based compound. Depending upon temperature ranges the conversion of the carbon nanotubes to carbide-based nanorods can be complete or partial. The rigid porous structure of carbide nanorods and/or carbon nanotubes can be further treated with an oxidizing agent to form oxycarbide nanorods and/or oxycarbides. The rigid porous structures of the invention can also be prepared from loose or aggregates of carbide-based nonorods and/or oxycarbide-based nanorods by initially forming a suspension in a medium, separating the suspension from the medium, and pyrolyzing the suspension to form rigid porous structures. The present invention also provides a process for making supported catalysts for selected fluid phase catalytic reactions.